The present invention relates to what is called a knock control system which detects knocking occurring in an internal combustion engine and controls knocking control factors such as ignition timing and an air-fuel ratio.
A technique for carrying out feedback control is conventionally known, in which feedback control a knock sensor operating as a knock detecting unit for detecting knocking and outputting an electric knocking detection signal is provided in a cylinder block of an internal combustion engine, a knock decision unit decides whether knocking has occurred or not in the internal combustion engine based on an output signal of the knock sensor, and, if the knock decision unit decides that knocking has occurred in the internal combustion engine, ignition timing, for example, is controlled to gradually shift from optimum ignition timing to the retardation side, and, if the knock decision unit decides that knocking has not occurred in the internal combustion engine, the ignition timing is controlled to gradually shift to the optimum ignition timing.
In this case, generally a decision on presence or absence of knocking is made by comparing an output signal of the knock sensor with a knock decision level V.sub.KD. Therefore, in order to make a knocking decision with high precision, it is important to set the knock decision level V.sub.KD to a suitable value.
Thus, the inventors of the present invention have already proposed a system for setting an optimum knock decision level, as disclosed, for example, in JP-A-60-243369 (U.S. Pat. No. 4,617,895). In this proposed system, a knock decision level V.sub.KD is calculated by multiplying a center value V.sub.BC of the distribution of output signals of a knock sensor by a value K determined according to a rotational speed number of an engine, for example, and the knock decision level V.sub.KD is automatically corrected based on the distribution pattern of logarithmic conversion value of the output signal of the knock sensor. More specifically, by using a correction value .DELTA.V based on the distribution pattern, the knock decision level V.sub.KD, is corrected to be decreased (V.sub.KD .rarw.V.sub.KD -.DELTA.V) or it is corrected to be increased (V.sub.KD .rarw.V.sub.KD +.DELTA.V), so that an optimum knock decision level may be set.
However, the setting of the correction value .DELTA.V is a problem in this case. If the correction value .DELTA.V is set to a larger value, the response characteristics of the knock decision level V.sub.KD (in other words, the followup capability of the knock decision level V.sub.KD at a transient time of the internal combustion engine, for example) can be obtained, but the stability of the knock decision level V.sub.KD, is deteriorated. On the other hand, if the correction value .DELTA.V is set to a smaller value, the stability of the knock decision level V.sub.KD can be obtained, but its response characteristics are deteriorated.
In other words, if the correction value .DELTA.V is set by laying weight on the stability, there is a risk that an erroneous decision is made that knocking has occurred even when knocking has not occurred, under such a condition that an output signal of the knock sensor has changed suddenly, for example, at the time of an accelerated speed of a vehicle (because the output signal becomes larger at the time of sudden acceleration). On the other hand, if the correction value .DELTA.V is set by laying weight on the response characteristics, there is a risk that a knocking decision can not be made at last, because a knock decision value is updated greatly under the influence of a noise even in a normal operating state of the engine.
In order to solve the above problems, the inventors of the present invention have proposed an apparatus for detecting a state of an internal combustion engine and increasing the correction value .DELTA.V thereby to raise the response characteristics during a predetermined time period only in an operating state of the engine where a delay in the response of the knock decision level V.sub.KD is a problem, as disclosed in JP-A-1-31564 (U.S. Pat. No. 4,993,387).
However, in the method of calculating the knock decision level V.sub.KD by using the above-proposed apparatus, a state of change of a knock sensor signal is assumed based on a state of change of a rotational speed number of the engine. For example, at the time of acceleration of the engine, the correction value .DELTA.V of the knock decision level V.sub.KD is set to a large value based on the fact that a knock sensor signal becomes large under such an accelerating condition, thereby improving the response characteristics. That is, the knock decision level V.sub.KD is not updated as a result of detecting a state of change of a knock sensor signal, but the knock decision level V.sub.KD is updated indirectly based on a value obtained by the assumption of a state of change of the knock sensor signal from a state of change of a rotational speed number of the engine.
According, it is very difficult to assume at what degree of change of a rotational speed number of the internal combustion engine the knock sensor signal would change greatly, because there arises dispersion in a knock sensor signal at every internal combustion engine. Further, the precision of knock detection will be degraded undesirably when the knock sensor signal per se does not change greatly as assumed, even when the rotational speed number of the engine has changed under a certain travelling condition of a vehicle.